While computing the x and y offsets, it's the video resolution and
resized overlay resolution to be used instead of actual overlay image
resoltuion. Due to this, the overlay image used to get wrongly overlayed
in undesired location
https://bugzilla.gnome.org/show_bug.cgi?id=757292

Set positioning-mode=pixels-absolute to allow positioning with
absolute coordinates, meaning negative x/y offsets will be
interpreted as being to the left/above the video frame instead
of being interpreted as relative to the right/bottom edge of
the video frame (which is a silly default, but that's how it is).
This means we can nicely slide images into and out of the frame,
see gdkpixbufoverlay-test.
https://bugzilla.gnome.org/show_bug.cgi?id=739566

packetized mode is being set when framerate is being set
which is not correct. Changing the same by checking the
input segement format. If input segment is in TIME it is
Packetized, and if it is in BYTES it is not.
https://bugzilla.gnome.org/show_bug.cgi?id=736252

Not sure why it as disabled exactly given that it had already
been ported (though without metas or baseclass).
Move plugin_init bits into separate source file, and rename
decoder element to gdkpixbufdec.